Inflammatory diseases, including arthritis, are often associated with reduced life expectancy, due in part to an excess of cardiovascular disease deaths. Low dose methotrexate (MTX) therapy is a mainstay for the long-term management of arthritis. Arthritis patients with cardiovascular disease who are treated with MTX have higher cardiovascular mortality rates than their peers treated with alternative disease-modifying anti-rheumatoid drugs (DMARDs). In addition, there is considerable inter-individual variation in the clinical efficacy of MTX, and about 30% of patents experience unacceptable toxicity. Objective methods to identify those for whom MTX therapy will be effective and minimally toxic, without greatly enhancing cardiovascular disease risk, would therefore contribute significantly to the clinical management of arthritis and other inflammatory conditions. MTX inhibits dihydrofolate reductase, an enzyme involved in purine synthesis and a component of the broader folate/Hcy metabolic axis. Methylenetetrahydrofolate reductase (MTHFR) is pivotal in controlling the distribution of folate derivatives between the two main constituent pathways that serve cellular methylation reactions and nucleic acid synthesis. In addition, methionine synthase (MTR), cystathionine Beta-synthase (CBS), and methionine synthase reductase (MTRR) are involved in reactions that control Hcy concentrations. Functional polymorphisms of MTHFR, MTR, CBS and MTRR significantly modify intracellular levels of folate derivatives and/or circulating Hcy levels, thereby increasing the risk of Hcy-associated pathologies. These polymorphisms, alone or in combination, may "prime" the folate/Hcy metabolic axis to respond to MTX by adopting an extreme pathogenic phenotype, and may also be significant determinants of the efficacy and toxicity associated with the drug. We will access the above in a pharmacogenetic analysis of 300 arthritis patients who are about to embark on MTX therapy. Pre-treatment and in-treatment folate derivative, B vitamin and Hcy concentrations will be determined together with MTHFR, MTR, CBS, and MTRR genotypes to establish whether there are particular phenotypic and/or genotypic variables that can be used to predict MTX efficacy and toxicity, and the likelihood of MTX-mediated enhancement of Hcy-associated disease risk. This research may establish the genetic parameters that mandate the treatment of arthritic patients with either MTX or an alternative DMARD. It has the potential to facilitate individualized treatment protocols that are less empirical and therefore more effective, and to reduce the incidence of cardiovascular co-morbidity.